At present, solid-state imaging devices including CMOS sensor are applied to various uses, such as digital still cameras, video movie cameras and monitor cameras. In particular, dominant devices are single-chip imaging devices which acquire plural color information by a single pixel array.
With a demand for an increase in the number of pixels and a decrease in optical size in recent years, there is a tendency that the pixel size is reduced more and more. For example, the pixel size of a CMOS sensor which has widely been used in recent years in digital cameras or the like is about 1.75 μm to about 2.8 μm. There is the following tendency with such fine pixels.
First, if the area of a unit pixel is decreased, the number of photons that can be received by the unit pixel decreases in proportion to the unit pixel area. As a result, the S (signal)/N (noise) ratio, relative to photon shot noise, decreases. If the S/N ratio cannot be maintained, the image quality on a reproduced screen deteriorates, and the quality of a reproduced image tends to lower.
Secondly, if the area of the unit pixel is reduced, crosstalk increases between neighboring pixels. As a result, although each pixel should normally have sensitivity only with respect to its unique wavelength region, the pixel has sensitivity with respect to a wavelength region at which the pixel should not normally have sensitivity. Consequently, color mixing occurs, and there is a tendency that the color reproducibility on the reproduced screen considerably deteriorates.
Thus, it is necessary to prevent a decrease in the S/N ratio by minimizing the decrease in sensitivity, thereby to maintain the S/N ratio even if the pixel is reduced in size, and it is necessary to prevent as much as possible the occurrence of color mixing, thereby to prevent degradation in color reproducibility even if the pixel is reduced in size.
As a structure for coping with the above-described tendency, there is known, for instance, a back-surface illumination type solid-state imaging device (see, e.g. Jpn. Pat. Appln. KOKAI Publication No. 2006-128392).
In the back-surface illumination type solid-state imaging device, incident light is radiated on a silicon (Si) surface (back surface) which is opposite to the silicon (Si) surface (front surface) on which a signal scanning circuit and its wiring layer are formed. In this back-surface illumination type structure in which light is incident on the silicon (Si) surface which is opposite to the silicon (Si) surface on which the signal scanning circuit and its wiring layer are formed, the light, which is incident on the pixel, can reach a light-receiving region, which is formed within the silicon (Si) substrate, without being blocked by the wiring layer. Thus, even with the fine pixel, a high quantum efficiency can be realized. As a result, with respect to the above-described first problem, that is, even in the case where the reduction in size of the pixel is progressed, there is a merit in suppressing the degradation in quality of the reproduced image.
In the conventional back-surface illumination type solid-state imaging device, however, no effective solution can be given to the above-described second problem. Specifically, in the back-surface illumination type solid-state imaging device, while incident light enters the silicon (Si) substrate that is the light-receiving region without being blocked by the signal scanning circuit and its wiring layer, there is a tendency that the incident light, which is not blocked by the wiring layer, leaks to a neighboring pixel, leading to color mixing.
For example, if the pixel is made finer, the aperture pitch of the micro-lens and color filter decreases, and diffraction occurs at a time point when the incident light falling on an R pixel with a particularly long wavelength has passed through the color filter. In this case, the light, which is obliquely incident on the light-receiving region in the silicon (Si) substrate, travels in a direction toward a neighboring pixel. If the light enters the neighboring pixel beyond an inter-pixel boundary, the light causes photoelectrons in the neighboring pixel, leading to crosstalk and color mixing. In addition, the light leaking into the light-receiving regions of the neighboring G pixel and B pixel causes color mixing. Hence, color reproducibility on the reproduced screen deteriorates, and the image quality decreases.